As the world's reserves of oil and gas run out over the coming decades, the birth-rates of societies are likely to fall considerably, a US scientist says.

According to some estimates, the global population may rise from its current 6.3bn today to almost 9bn by 2050.

But Virginia Abernethy told a Seattle meeting that the loss of fossil fuels would hit world economies very hard.

"Economic hardship discourages people from marrying young and from having closely spaced children," she said.

The anthropologist and professor emerita of psychiatry from Vanderbilt University was speaking here in Washington State at the annual gathering of the American Association for the Advancement of Science.

"The availability of energy has been a major factor in population growth," said Professor Abernethy.

"In the modern context, energy use per capita affects economic activity. So a prolonged decline in energy use per capita will tend to depress the economy which, in turn, will cause a decline in the fertility rate."

Dual role

Abernethy said fossil fuels had become fundamental to the continued economic growth and improving standards of living which many societies had witnessed in recent decades.

Not only does petroleum provide the fuel that powers modern vehicles and the natural gas that people use for home heating and cooking, but petroleum products are also the source for hundreds of industrial and agricultural products, including fertilisers, pesticides and plastics.

This meant that petroleum could not be easily replaced by other fuels and feed stocks, the professor argued.

It describes how people increase the size of their families when they are convinced that economic opportunities are expanding and rein back on their objectives when they believe resources are shrinking and the difficulty of raising children is increasing.

"In the US, we saw a huge effect from higher energy prices after the 1974 Opec energy embargo, which caused a fairly severe recession, and that was followed by quite sharp declines in the fertility rates of American whites and blacks."

The effects of the coming changes were already being seen, said the professor.

The average number of births per woman over a lifetime was now 2.3 and falling, she added.

"I think it is a good thing that we limit family size because that could stabilise the global population and help avoid the greater catastrophe that would come if we push our system right to the limits and have to endure very high mortality rates."